Optical module assembly, optical module, package for optical module and flexible printed board

ABSTRACT

A package for an optical module includes a substrate that includes a first wiring layer, a second wiring layer, and a third wiring layer. The package includes a first insulating layer between the first wiring layer and the second wiring layer, the first insulating layer including first vias. The package includes a second insulating layer between the second wiring layer and the third wiring layer, the second insulating layer including second vias and third vias. Each first vias is provided between a corresponding second via and a corresponding third via. The first vias are arranged at a first interval along a first direction. The second vias are arranged at a second interval along the first direction. Each second vias is disposed at an offset by half of the second interval from the corresponding third via.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. § 119 to JapanesePatent Application No. 2022-082044, filed May 19, 2022, the contents ofwhich are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to a package for optical module, anoptical module, a flexible printed board, and an optical moduleassembly.

BACKGROUND

Japanese Unexamined Patent Application Publication No. 2015-015513discloses a flexible circuit board that has a signal line wiringconductor and a ground line wiring conductor. The signal line wiring isprovided on an upper surface of the flexible circuit board, and a groundline wiring conductor is provided on a bottom surface of the flexiblecircuit board. The signal line wiring conductor and the ground linewiring conductor are opposed to each other. In the flexible circuitboard, via hole conductors for electrically connecting the signal linewiring conductor and the ground line wiring conductor are formed atequal pitch intervals in the longitudinal direction.

Japanese Unexamined Patent Application Publication No. 2010-192767discloses a semiconductor device in which a semiconductor element ismounted on an upper surface of a wiring substrate. In the wiringsubstrate, an interlayer connection via hole that is connected to aground is arranged between an interlayer connection via hole, which isconnected to a signal terminal in the second column, and an interlayerconnection via hole, which is connected to a signal terminal in thefourth column. The interlayer connection via hole connected to theground serves as a shield to suppress crosstalk between signals.

SUMMARY

The present disclosure provides a package for an optical module. Thepackage includes a side wall and a substrate provided through the sidewall in a first direction, the substrate including a first wiring layerthat is externally exposed, and the first wiring layer including a firstsignal terminal extending along the first direction, a second signalterminal extending along the first direction, and a first groundterminal extending along the first direction. The first ground terminalis provided between the first signal terminal and the second signalterminal, when viewed in a second direction perpendicular to the firstdirection. The package includes a second wiring layer disposed under thefirst wiring layer, the second wiring layer including a first groundpattern, the first ground pattern being electrically coupled to thefirst ground terminal via a plurality of first vias. The packageincludes a third wiring layer disposed under the second wiring layer,the third wiring layer including a second ground layer electricallycoupled to the first ground pattern via a plurality of second vias and aplurality of third vias. The package includes a first insulating layerdisposed between the first wiring layer and the second wiring layer, thefirst insulating layer including the plurality of first vias, theplurality of first vias being arranged at a first interval along thefirst direction. The package includes a second insulating layer disposedbetween the second wiring layer and the third wiring layer. The secondinsulating layer includes the plurality of second vias, the second viasbeing arranged at a second interval along the first direction, andincludes the plurality of third vias, the third vias being arranged atthe second interval along the first direction. When viewed in the seconddirection, each first via of the plurality of first vias is providedbetween a corresponding second via among the plurality of second viasand a corresponding third via among the plurality of third vias. Whenviewed in the first direction, each second via of the plurality ofsecond vias is disposed at an offset by half of the second interval fromthe corresponding third via among the plurality of third vias.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a configuration of an opticaltransceiver according to an embodiment of the present disclosure.

FIG. 2 is a perspective view of an optical module assembly according toan embodiment of the present disclosure.

FIG. 3 is a perspective view of an optical module according to anembodiment of the present disclosure.

FIG. 4 is a top view of a flexible printed board according to theembodiment of the present disclosure.

FIG. 5 is a bottom view of a flexible printed board according to anembodiment of the present disclosure.

FIG. 6 is a top view of a terminal portion of an optical moduleaccording to an embodiment of the present disclosure.

FIG. 7 is a perspective view of a substrate of an optical moduleaccording to an embodiment of the present disclosure.

FIG. 8 illustrates a terminal portion of an optical module according toan embodiment of the present disclosure.

FIG. 9 illustrates a terminal portion of an optical module according toan embodiment of the present disclosure.

FIG. 10 illustrates a terminal portion of an optical module according toan embodiment of the present disclosure.

FIG. 11 illustrates an electric field distribution of an optical moduleaccording to an embodiment of the present disclosure.

FIG. 12 illustrates an electric field distribution of an optical moduleof a comparative example.

FIG. 13A illustrates an arrangement of via holes connecting a groundpattern of a second wiring layer and a ground pattern of a third wiringlayer.

FIG. 13B illustrates an arrangement of via holes connecting a groundpattern of a second wiring layer and a ground pattern of a third wiringlayer.

DETAILED DESCRIPTION Details of Embodiments of Present Disclosure

Specific examples of an optical module of the present disclosure will bedescribed below with reference to the drawings. The present invention isnot limited to these examples, but is defined by the scope of claims andis intended to include all modifications within the meaning and scopeequivalent to the scope of claims.

In the specification and the drawings of each embodiment, configurationelements having substantially the same or corresponding functions aredenoted by the same reference numerals, and redundant descriptionthereof may be omitted. For ease of understanding, the scale of eachpart in the drawings may be different from the actual scale.

In directions such as parallel, perpendicular, orthogonal, horizontal,and vertical directions, deviations are allowed to such an extent thatthe effects of the embodiment are not impaired. The shape of the cornerportion is not limited to a right angle and may be rounded in an arcuateshape. Parallel, perpendicular, orthogonal, horizontal, and vertical mayinclude substantially parallel, substantially perpendicular,substantially orthogonal, substantially horizontal, and substantiallyvertical. For example, “substantially parallel” means that even if twolines or two surfaces are not completely parallel to each other, theycan be treated as being parallel to each other within an allowable rangein manufacturing. As in a case of the “substantially parallel,” theother substantially right angle, substantially orthogonal, substantiallyhorizontal, and substantially vertical are intended to apply to each ofthem as long as a mutual positional relationship between two lines ortwo surfaces is within a range allowed in manufacturing.

An optical transceiver 1 according to an embodiment of the presentdisclosure will be described. FIG. 1 schematically illustrates aconfiguration of the optical transceiver 1 according to an embodiment ofthe present disclosure.

The optical transceiver 1 includes optical modules 10T and 10R, flexibleprinted boards 20T and 20R, and a circuit board 30. The optical modules10T and 10R are examples of optical modules 10 according to anembodiment of the present disclosure.

(Optical Module)

The optical modules 10T and 10R mutually convert electrical signals andoptical signals. The optical module 10T is, for example, a transmitteroptical subassembly (TOSA). The optical module 10R is, for example, areceiver optical subassembly (ROSA).

The optical module 10T includes, for example, a driver 11 and atransmitting optical element 12. The transmitting optical element 12 is,for example, a light emitting element or an optical modulator.

For example, the driver 11 (drive circuit) outputs a drive signal Td fordriving the transmitting optical element 12 to the transmitting opticalelement 12 based on a transmission signal Tx2 from a digital signalprocessor (DSP) 31 included in the circuit board 30.

The transmitting optical element 12 outputs a transmission opticalsignal Lt in response to the driving signal Td. The transmission opticalsignal Lt output from the transmitting optical element 12 is transmittedto another optical transceiver via an optical fiber. The transmittingoptical element 12 is, for example, a laser diode or a Mach-Zehndermodulator.

The optical module 10R includes, for example, a transimpedance amplifier13 and a receiving optical element 14. The transimpedance amplifier 13converts a received signal Ri, which is a current signal from thereceiving optical element 14, into a received signal Rx2, which is avoltage signal, and outputs the received signal Rx2 to the DSP 31 of thecircuit board 30.

The receiving optical element 14 is for example a photodiode. Thereceiving optical element 14 converts a received optical signal Lrreceived from another optical transceiver via an optical fiber into thereceived signal Ri.

The optical transceiver 1 may further include a light source 15 (notshown). The light source 15 is, for example, a wavelength tunable laser.The light source 15 generates continuous light (CW light) Lb having apredetermined peak wavelength, and outputs to the transmitting opticalelement 12. For example, when the transmitting optical element 12 is anoptical modulator, the CW light Lb supplied from the light source 15 ismodulated according to the drive signal Td to generate the transmissionoptical signal Lt. The receiving optical element 14 may include a 90°optical hybrid. The receiving optical element 14 may generate thereceived signal Ri from an optical signal generated by interfering thereceived optical signal Lr with the CW light Lb.

(Flexible Printed Board)

The flexible printed board 20T electrically connects the optical module10T and the circuit board to each other. The flexible printed board 20Relectrically connects the optical module 10R and the circuit board 30.Each of the flexible printed boards 20T and 20R includes a plurality ofsignal wirings. The plurality of signal wirings form, for example, atransmission line. For example, each of the flexible printed boards 20Tand 20R includes a differential signal wiring including a pair of signalwirings to transmit one differential signal. The differential signalwiring constitutes, for example, a differential transmission line inorder to transmit a high-speed differential signal with good waveformquality.

(Circuit Board)

The circuit board 30 includes, for example, a control circuit thatcontrols the inside of the optical transceiver 1. For example, thecontrol circuit includes a detection circuit, a microcontroller, and thelike for controlling the operation of the optical modules 10T and 10R.The circuit board 30 performs communication for monitoring andcontrolling the optical transceiver 1 with a host (transmission device)to which the optical transceiver 1 is connected. Furthermore, thecircuit board 30 controls the optical modules 10T and 10R based on, forexample, communication with the host. The circuit board 30 transmitsinformation on the operation states of the optical modules 10T and 10Rto the host in response to an inquiry from the host.

The circuit board 30 includes, for example, a DSP 31. The DSP 31converts a transmission signal Tx1 transmitted from the host into atransmission signal Tx2, which is to be transmitted to the driver 11.For example, in a case where the DSP 31 includes a clock data recovery(CDR) circuit, a waveform-shaped signal is generated from thetransmission signal Tx1 and is output as the transmission signal Tx2. Inaddition, the DSP 31 converts the received signal Rx2, which is from thetransimpedance amplifier 13, into a received signal Rx1, which is to betransmitted to the host. For example, when the DSP 31 includes a CDRcircuit, a waveform-shaped signal is generated from the received signalRx2 and is output as the received signal Rx1. For example, the DSP 31transmits and receives a control signal Ctl1 for monitor controlling, toand from the host, respectively, via a dedicated signal wiring that isprovided separately from the signal wirings for transmitting thetransmission signal Tx1 and the received signal Rx1.

Next, the configuration used in the optical transceiver 1 will bedescribed in detail. In the drawings, an XYZ orthogonal coordinatesystem may be shown for convenience of description. For example, withrespect to a coordinate axis perpendicular to the paper surface of thedrawing, when an X mark is shown in a circle of the coordinate axis, itindicates that the direction toward the back with respect to the papersurface is a positive region of the coordinate axis. In addition,regarding a coordinate axis perpendicular to the paper surface of thedrawing, when a black circle is shown in a circle of the coordinateaxis, this indicates that the front side with respect to the papersurface is a positive region of the coordinate axis. However, thecoordinate system mainly indicates directions for the purpose ofexplanation, and does not limit each of the coordinates and attitude ofthe optical module or the like of the present disclosure.

In the present disclosure, unless otherwise specified, the X-axis is thedirection in which the terminals of the optical modules 10T and 10R arealigned, the Y-axis is the thickness direction of the terminals of theoptical modules 10T and 10R, and the Z-axis is the direction in whichthe terminals of the optical modules 10T and 10R extend. For example,the transmission signal Tx2 and the received signal Rx2 are transmittedbetween the circuit board 30 and the optical modules 10T and 10R alongthe Z-axis. In addition, unless otherwise specified, coordinate axeshaving the same name shown in each drawing represent the same thing. Forexample, the X-axis in FIG. 2 represents the same as the X-axis in FIG.3 .

(Optical Module Assembly)

An optical module assembly 2 includes the optical module 10T and theflexible printed board 20T. Alternatively, the optical module assembly 2may include the optical module 10R and the flexible printed board 20R.Hereinafter, a case where the optical module assembly 2 is configured bythe optical module 10T and the flexible printed board 20T will bedescribed. FIG. 2 is a perspective view of the optical module assembly 2according to an embodiment of the present disclosure. The optical module10T and the flexible printed board 20T included in the optical moduleassembly 2 are electrically connected to each other by soldering or thelike.

(Optical Module)

The optical module 10T includes a package 16 in addition to the driver11 and the transmitting optical element 12. FIG. 3 is a perspective viewof the optical module 10T according to an embodiment of the presentdisclosure. For example, the driver 11 and the transmitting opticalelement 12 are housed in the package 16.

The package 16 has, for example, a rectangular parallelepiped outershape. More specifically, the package 16 includes a substrate 16 a, abase plate 16 b, a side wall 16 c, and a lid 16 d.

The substrate 16 a introduces, for example, a signal from the outside ofthe optical module 10T into the optical module 10T. The substrate 16 amay output a signal from the inside of the optical module 10T to theoutside of the optical module 10T. The substrate 16 a has signal wiringsfor transmitting signals between the inside and the outside of thepackage 16. Such a signal wiring is also referred to as a feedthrough.The substrate 16 a having such signal wirings may be referred to as afeed-through.

The substrate 16 a is, for example, a ceramic substrate. The substrate16 a is a multilayer wiring substrate having a wiring layer on thesurface and inside thereof. The substrate 16 a includes, for example,wiring layers and an insulating layer formed of ceramic between thewiring layers. The optical module 10T has a terminal portion 10 a on thesubstrate 16 a. The wiring layer is provided with signal wirings fortransmitting the transmission signal Tx2 and the received signal Rx2, aground line, a power supply line for supplying power to the inside, andthe like. The terminal portion 10 a is provided in, for example, awiring layer (first wiring layer) on a surface of the substrate 16 a.

The base plate 16 b has, for example, a rectangular plate-like outershape. The base plate 16 b has, for example, an upper surface parallelto the XZ plane. For example, the driver 11 and the transmitting opticalelement 12 are mounted on the upper surface of the base plate 16 b. Thebase plate 16 b closes the lower side (−Y side) of the side wall 16 c.The base plate 16 b is, for example, bonded to the lower side of theside wall 16 c or integrally formed with the side wall 16 c.

The side wall 16 c forms a side wall of the package 16. The side wall 16c includes a pair of side walls (hereinafter referred to as lateralwalls) parallel to the YZ plane and a pair of side walls (hereinafterreferred to as a front wall and a rear wall) parallel to the XY plane.

A front wall 16 cf is provided with the substrate 16 a. The rear wall islocated opposite to the front wall 16 cf in the Z-axis direction. Therear wall is provided with, for example, an optical component (notshown) for outputting the transmission optical signal Lt to the outside.Therefore, for example, the transmission signal Tx2 is input to thefront wall 16 cf, and the transmission optical signal Lt is output fromthe rear wall. The front wall 16 cf and the rear wall are connected tothe pair of lateral walls, respectively. The pair of lateral walls, thefront wall 16 cf, and the rear wall form a space (internal space) insidethe package 16. The internal space is surrounded by the pair of lateralwalls, the front wall 16 cf, and the rear wall in a plan view of the XZplane. The driver 11 and the transmitting optical element 12 areaccommodated in the internal space. When the circuit board 30 and theoptical module 10T are electrically connected to each other via theflexible printed board 20T, the transmission signal Tx2 generated by thecircuit board 30 is transmitted to the optical module 10T. A tip of theflexible printed board 20T is connected to the substrate 16 a of theoptical module 10T, and the transmission signal Tx2 is input to thedriver 11 inside the package via the substrate 16 a. When thetransmission signal Tx2 is configured with four channels, the waveformquality of the transmission optical signal Lt is improved by suppressingcrosstalk between the channels (the channels will be described later).Further, since the circuit board 30 and the optical module 10R areelectrically connected to each other through the flexible printed board20R, the received signal Rx2 generated by the optical module 10R inresponse to the received optical signal Lr is transmitted to the circuitboard 30. When the received signal Rx2 is configured with four channels,reception performance of the received optical signal Lr is improved bysuppressing crosstalk between the channels.

The lid 16 d is connected to an upper side (+Y side) of the side wall 16c. For example, the lid 16 d is bonded to the upper side of the sidewall 16 c. The lid 16 d is, for example, a metal lid and is joined to anupper portion of the side wall 16 c by seam welding. The internal spaceis defined by the substrate 16 a, the base plate 16 b, the side wall 16c, and the lid 16 d. For example, the package 16 is hermetically sealedso that a gas (for example, an inert gas) in the internal space isisolated from the outside of the package 16. The optical module 10R hasa package similar to the package 16 of the optical module 10T, andaccommodates the transimpedance amplifier 13 and the receiving opticalelement 14 in the internal space of the package. Description of thepackage of the optical module 10R will be omitted.

(Flexible Printed Board)

The configuration of the flexible printed board 20T will be described indetail. FIG. 4 is a top view of the flexible printed board 20T accordingto an embodiment of the present disclosure. Specifically, FIG. 4 is atop view of the flexible printed board 20T when viewed from the positiveregion of the Y-axis toward the negative region of the Y-axis (which maybe hereinafter referred to as “in a plan view of the upper surface”).FIG. 5 is a bottom view of the flexible printed board 20T according toan embodiment of the present disclosure. Specifically, FIG. 5 is a topview of the flexible printed board 20T when viewed from the negativeregion of the Y-axis toward the positive region of the Y-axis (which maybe hereinafter referred to as “in a plan view of the lower surface”).The flexible printed board 20R has the configuration similar to that ofthe flexible printed board 20T. The flexible printed board 20R will notbe described in detail.

The flexible printed board 20T extends along the Z-axis direction. Forexample, the flexible printed board 20T has similar shapes at both endsthat are situated in the extending direction (Z-axis direction) of theflexible printed board 20T. Note that the shapes at both ends may differfrom each other. In FIGS. 4 and 5 , the end portion of the flexibleprinted board 20T on a side that is connected to the optical module 10T(the negative side of the Z-axis) is shown, and the description of theend portion of the flexible printed board 20T on a side (the positiveside of the Z-axis) opposite to the side that is connected to theoptical module 10T is omitted.

The flexible printed board 20T has a pair of signal wiring WaS1 p andsignal wiring WaS1 n on an upper surface LmS1 of an insulating layer Lm.The pair of signal wiring WaS1 p and signal wiring WaS1 n is formed aspart of the wiring layer of the upper surface. Each of the signal wiringWaS1 p and the signal wiring WaS1 n extends along the Z-axis direction.A differential signal (for example, the transmission signal Tx2) istransmitted to the signal wiring WaS1 p and the signal wiring WaS1 n.The signal wiring WaS1 p and the signal wiring WaS1 n are configured bya material having conductivity. For example, the signal wiring WaS1 pand the signal wiring WaS1 n are configured by a metal such as copper(Cu). Other signal wirings described below are also formed of aconductive material. For example, when the differential signal includesa positive-phase component (positive-phase signal) and a negative-phasecomponent (negative-phase signal), the positive-phase signal istransmitted by the signal wiring WaS1 p, and the negative-phase signalis transmitted by the signal wiring WaS1 n. The signal wiring WaS1 p andthe signal wiring WaS1 n are arranged adjacent to each other along, forexample, the X-axis direction. The signal wiring WaS1 p and the signalwiring WaS1 n are configured as a transmission line, for example.

The flexible printed board 20T has a signal terminal FaS1 p connected tothe signal wiring WaS1 p at an end portion of the upper surface LmS1 ofthe insulating layer Lm on the side connected to the optical module 10T.The flexible printed board 20T has a signal terminal FaS1 n connected tothe signal wiring WaS1 n at the end portion of the upper surface LmS1 onthe side connected to the optical module 10T. The signal terminal FaS1 pand the signal terminal FaS1 n may be collectively referred to as adifferential signal terminal FaS1. The signal terminal FaS1 p and thesignal terminal FaS1 n are configured by a material having conductivity.For example, the signal terminal FaS1 p and the signal terminal FaS1 nare configured by the same metal as the metal configuring the signalwiring WaS1 p and the signal wiring WaS1 n. The signal terminal FaS1 pand the signal terminal FaS1 n are arranged adjacent to each other, forexample, along the X-axis direction.

Similarly, the flexible printed board 20T has a pair of signal wiringWaS2 p and signal wiring WaS2 n on the upper surface LmS1 of theinsulating layer Lm. The pair of signal wiring WaS2 p and signal wiringWaS2 n is formed as part of the wiring layer of the upper surface. Eachof the signal wiring WaS2 p and the signal wiring WaS2 n extends alongthe Z-axis direction. Differential signals different from thedifferential signals transmitted by the signal wiring WaS1 p and thesignal wiring WaS1 n are transmitted in the signal wiring WaS2 p and thesignal wiring WaS2 n. The signal wiring WaS2 p and the signal wiringWaS2 n are formed as, for example, a transmission line. The flexibleprinted board 20T has a signal terminal FaS2 p connected to the signalwiring WaS2 p, and has a signal terminal FaS2 n connected to the signalwiring WaS2 n. The signal terminal FaS2P and the signal terminal FaS2 nare at an end portion of the upper surface LmS1 on the side that isconnected to the optical module 10T. The signal terminal FaS2 p and thesignal terminal FaS2 n may be collectively referred to as a differentialsignal terminal FaS2.

Further, the flexible printed board 20T includes a pair of signal wiringWaS3 p and signal wiring WaS3 n, and includes a pair of signal wiringWaS4 p and signal wiring WaS4 n. These pairs are on the upper surfaceLmS1. The pair of signal wiring WaS3 p and signal wiring WaS3 n has theconfiguration similar to that of the pair of signal wiring WaS1 p andsignal wiring WaS1 n. The pair of signal wiring WaS4 p and signal wiringWaS4 n has the configuration similar to that of the pair of signalwiring WaS1 p and signal wiring WaS1 n. The pair of signal wiring WaS3 pand signal wiring WaS3 n and the pair of signal wiring WaS4 p and signalwiring WaS4 n will not be described in detail.

Further, the flexible printed board 20T includes a signal terminal FaS3p connected to the signal wiring WaS3 p, a signal terminal FaS3 nconnected to the signal wiring WaS3 n, a signal terminal FaS4 pconnected to the signal wiring WaS4 p, and a signal terminal FaS4 nconnected to the signal wiring WaS4 n. Detailed description of thesignal terminal FaS3 p, the signal terminal FaS3 n, the signal terminalFaS4 p, and the signal terminal FaS4 n will be omitted.

The signal wiring WaS1 p, signal wiring WaS1 n, signal wiring WaS2 p,signal wiring WaS2 n, signal wiring WaS3 p, signal wiring WaS3 n, signalwiring WaS4 p, and signal wiring WaS4 n are arranged on the uppersurface LmS1 in order from the negative side of the X-axis, when viewedin the X-axis direction. The flexible printed board 20T has aninsulating coating Lga on the upper surface LmS1 so as to cover thesignal wiring WaS1 p, the signal wiring WaS1 n, and other signalwirings. The insulating coating Lga is formed of a material having aninsulating property, and prevents, for example, each signal wiring fromelectrically contacting another wiring or the like and prevents thesignal wiring from being damaged due to contact with the outside. Theinsulating coating Lga may be, for example, a coverlay or a solderresist.

The flexible printed board 20T has a ground pattern FaG1 at the endportion on the negative side of the X-axis at the end on the sideconnected to the optical module 10T. The ground pattern FaG1 isconfigured by a material having conductivity. For example, the groundpattern FaG1 is configured by a metal such as Cu. When the opticalmodule 10T is mounted in the optical transceiver 1, the ground patternFaG1 is electrically connected to the ground wiring of the opticaltransceiver 1. Note that other ground patterns described below areconfigured in the manner similar to the ground pattern FaG1. Inaddition, the flexible printed board 20T has a ground pattern FaG2between the signal terminal FaS1 n and the signal terminal FaS2 p, inother words, between the differential signal terminal FaS1 and thedifferential signal terminal FaS2. Similarly, the flexible printed board20T has a ground pattern FaG3 between the signal terminal FaS2 n and thesignal terminal FaS3 p, in other words, between the differential signalterminal FaS2 and the differential signal terminal FaS3. In addition,the flexible printed board 20T has a ground pattern FaG4 between thesignal terminal FaS3 n and the signal terminal FaS4 p, in other words,between the differential signal terminal FaS3 and the differentialsignal terminal FaS4. A ground pattern FaG5 is provided at the endportion on the positive side of the X-axis at the end portion on theside connected to the optical module 10T.

The flexible printed board 20T has a ground pattern WbG on a lowersurface LmS2 of the insulating layer Lm. The ground pattern WbG is aplanar pattern that extends in the XZ-plane parallel to both the X-axisdirection and the Z-axis direction. The ground pattern WbG is formed asa so-called solid pattern, for example. The ground pattern WbG is formedas a part of the wiring layer of the lower surface LmS2. In addition,the flexible printed board 20T has a ground terminal FbG1, a groundterminal FbG2, a ground terminal FbG3, a ground terminal FbG4, and aground terminal FbG5 in order from the negative side of the X-axis alongthe X-axis direction at the end portion of the lower surface LmS2 of theinsulating layer Lm on the side connected to the optical module 10T.Each of the ground terminal FbG1, ground terminal FbG2, ground terminalFbG3, ground terminal FbG4, and ground terminal FbG5 extends along theZ-axis direction and is connected to the ground pattern WbG. Aconfiguration including the ground terminal FbG1, the signal terminalFbS1 p, the signal terminal FbS1 n, and the ground terminal FbG2arranged along the X-axis direction is also referred to as a GSSGconfiguration. Here, S denotes a signal wiring (signal terminal) and Gdenotes a ground wiring (ground terminal).

The ground pattern FaG1 is electrically connected to the ground terminalFbG1 via a through hole via (hereinafter, simply referred to as a viahole). The circles shown in FIGS. 4 and 5 represent via holes. The viahole passes through the insulating layer Lm along the Y-axis direction,and is internally plated to electrically connect the wiring layer of theupper surface LmS1 and the wiring layer of the lower surface LmS2. Notethat the via hole may be filled with a metal. Similarly, the groundpattern FaG2 is electrically connected to the ground terminal FbG2through a via hole. Similarly, the ground pattern FaG3, the groundpattern FaG4, and the ground pattern FaG5 are electrically connected tothe ground terminal FbG3, the ground terminal FbG4, and the groundterminal FbG5, respectively, through the via holes.

The flexible printed board 20T includes the signal terminal FbS1 p andthe signal terminal FbS1 n between the ground terminal FbG1 and theground terminal FbG2. The signal terminal FbS1 p is electricallyconnected to the signal terminal FaS1 p through a via hole. The signalterminal FbS1 n is electrically connected to the signal terminal FaS1 nthrough a via hole.

Similarly, the flexible printed board 20T has a signal terminal FbS2 pand a signal terminal FbS2 n between the ground terminal FbG2 and theground terminal FbG3. The signal terminal FbS2 p is electricallyconnected to the signal terminal FaS2 p through a via hole. The signalterminal FbS2 n is electrically connected to the signal terminal FaS2 nthrough a via hole.

In addition, the flexible printed board 20T includes a signal terminalFbS3 p and a signal terminal FbS3 n between the ground terminal FbG3 andthe ground terminal FbG4. The signal terminal FbS3 p is electricallyconnected to the signal terminal FaS3 p through a via hole. The signalterminal FbS3 n is electrically connected to the signal terminal FaS3 nthrough a via hole.

Further, the flexible printed board 20T includes a signal terminal FbS4p and a signal terminal FbS4 n between the ground terminal FbG4 and theground terminal FbG5. The signal terminal FbS4 p is electricallyconnected to the signal terminal FaS4 p through a via hole. The signalterminal FbS4 n is electrically connected to the signal terminal FaS4 nthrough a via hole.

The flexible printed board 20T has an insulating coating Lgb on thelower surface LmS2 so as to cover the ground pattern WbG. The insulatingcoating Lgb is formed of a material having an insulating property andprevents, for example, the ground pattern from electrically contactinganother wiring or the like or the ground pattern from being damaged dueto contact with the outside. The insulating coating Lgb may be, forexample, a coverlay or a solder resist. Note that the insulating coatingLgb may cover portions other than portions where the wiring layer needsto be exposed, such as the signal terminal FbS1 p and the signalterminal FbS1 n.

In the flexible printed board 20T, the signal wiring WaS1 p, the signalwiring WaS1 n, and the ground pattern WbG configure a transmission line.Similarly, the signal wiring WaS2 p and signal wiring WaS2 n, the signalwiring WaS3 p and signal wiring WaS3 n, the signal wiring WaS4 p andsignal wiring WaS4 n, and the ground pattern WbG constitute transmissionlines. The transmission lines are configured as differentialtransmission lines, which are particularly suitable for transmittingdifferential signals. The transmission line may be, for example, amicrostrip line or a grounded coplanar line.

The plurality of ground terminals and the plurality of signal terminalsformed on the lower surface LmS2 of the flexible printed board 20T areconnected to the ground terminals and the plurality of signal terminalsformed on the terminal portion 10 a of the optical module 10T bysoldering. At this time, the ground terminal of the flexible printedboard 20T is connected to the ground terminal of the optical module 10Tin one-to-one correspondence, and the plurality of signal terminals ofthe flexible printed board 20T are connected to the plurality of signalterminals of the optical module 10T corresponding thereto in one-to-onecorrespondence.

When the flexible printed board 20T and the optical module 10T areconnected to each other, in order to reduce signal reflection at theconnection portion, the flexible printed board 20T and the opticalmodule 10T are preferably connected to each other such that impedance ismatched at the connection portion. Note that the plurality of groundterminals and the plurality of signal terminals formed on the uppersurface LmS1 of the flexible printed board 20T may be connected to thecorresponding ground terminals and the plurality of signal terminalsformed on the terminal portion 10 a of the optical module 10T bysoldering in a state where the flexible printed board 20T is turnedupside down (the upper surface LmS1 and the lower surface LmS2 arereplaced with each other). When the flexible printed board 20T is turnedupside down, the order of the differential signal terminals FaS1, FaS2,FaS3, and FaS4 in the X-axis direction is opposite to that beforeturning upside down, but the order of the differential signals may beappropriately changed so that the plurality of signal terminals of theflexible printed board 20T and the plurality of signal terminals of theoptical module 10T correctly correspond to each other.

<Details of Substrate 16 a>

The substrate 16 a configuring the terminal portion 10 a of the opticalmodule 10T will be described in detail. FIG. 6 is a top view of theterminal portion 10 a of the optical module 10T according to anembodiment of the present disclosure. Specifically, FIG. 6 is a top viewwhen the optical module 10T is viewed from the positive region of theY-axis toward the negative region of the Y-axis (or in a plan view ofthe substrate 16 a).

The optical module 10T includes a ground terminal TaG1, a signalterminal TaS1 p, a signal terminal TaS1 n, and a ground terminal TaG2 inthe terminal portion 10 a, more specifically, on an upper surface 16 a 5of the substrate 16 a, in order from the end on the negative side of theX-axis toward the positive side of the X-axis. The ground terminal TaG1,the signal terminal TaS1 p, the signal terminal TaS1 n, and the groundterminal TaG2 are formed as part of the wiring layer of the uppersurface 16 a 5. The wiring layer is configured by a material havingconductivity. For example, the wiring layer is constituted by a metalsuch as tungsten (W). Each of the signal terminal TaS1 p and the signalterminal TaS1 n extends along the Z-axis direction. The signal terminalTaS1 p and the signal terminal TaS1 n may be collectively referred to asa differential signal terminal TaS1. The configuration formed by theground terminal TaG1, the signal terminal TaS1 p, the signal terminalTaS1 n, and the ground terminal TaG2 arranged along the X-axis directionis an example of the GSSG configuration.

In addition, the optical module 10T includes a signal terminal TaS2 p, asignal terminal TaS2 n, and a ground terminal TaG3 adjacent to theground terminal TaG2 in this order toward the positive side of theX-axis on the upper surface 16 aS. Each of the signal terminal TaS2 pand the signal terminal TaS2 n extends along the Z-axis direction. Thesignal terminal TaS2 p and the signal terminal TaS2 n may becollectively referred to as a differential signal terminal TaS2. Forexample, when the differential signal includes a positive-phasecomponent (positive-phase signal) and a negative-phase component(negative-phase signal), the positive-phase signal is input to thesignal terminal TaS2 p, and the negative-phase signal is input to thesignal terminal TaS2 n. The configuration formed by the ground terminalTaG2, the signal terminal TaS2 p, the signal terminal TaS2 n, and theground terminal TaG3 arranged along the X-axis direction is an exampleof the GSSG configuration. As described above, this GSSG configurationand the above-described GSSG configuration may be configured to share aground terminal (G) at an end. Therefore, for example, it may beconfigured to be written as GSSGSSG.

Further, the optical module 10T has, on the upper surface 16 aS, asignal terminal TaS3 p, a signal terminal TaS3 n, and a ground terminalTaG4 adjacent to the ground terminal TaG3 in this order toward thepositive side of the X-axis. Each of the signal terminal TaS3 p and thesignal terminal TaS3 n extends along the Z-axis direction. The signalterminal TaS3 p and the signal terminal TaS3 n may be collectivelyreferred to as a differential signal terminal TaS3. The configuration ofthe ground terminal TaG3, the signal terminal TaS3 p, the signalterminal TaS3 n, and the ground terminal TaG4 arranged along the X-axisdirection is an example of the GSSG configuration. The configuration ofthe ground terminal TaG3, the signal terminal TaS3 p, the signalterminal TaS3 n, and the ground terminal TaG4 arranged along the X-axisdirection may have the same shape, interval, or the like as theconfiguration of the ground terminal TaG2, the signal terminal TaS2 p,the signal terminal TaS2 n, and the ground terminal TaG3.

Furthermore, the optical module 10T has, on the upper surface 16 aS, asignal terminal TaS4 p, a signal terminal TaS4 n, and a ground terminalTaG5 adjacent to the ground terminal TaG4 in this order toward thepositive side of the X-axis. Each of the signal terminal TaS4 p and thesignal terminal TaS4 n extends along the Z-axis direction. The signalterminal TaS4 p and the signal terminal TaS4 n may be collectivelyreferred to as a differential signal terminal TaS4. The configuration ofthe ground terminal TaG4, the signal terminal TaS4 p, the signalterminal TaS4 n, and the ground terminal TaG5 arranged along the X-axisdirection is an example of the GSSG configuration.

Each of the ground terminal TaG1, the ground terminal TaG2, the groundterminal TaG3, the ground terminal TaG4 and the ground terminal TaG5extends along the Z-axis direction on the upper surface 16 aS. Note thatthe length (width) in the X-axis direction, the interval (pitch) in theX-axis direction, and the length in the Z-axis direction of each groundterminal may be set to the same value. For example, the interval of theground terminal TaG1 and the ground terminal TaG2 may be set to the samevalue as the interval of the ground terminal TaG2 and the groundterminal TaG3. Each of the ground terminal TaG1, the ground terminalTaG2, the ground terminal TaG3, the ground terminal TaG4, and the groundterminal TaG5 is electrically connected to the ground wiring inside thepackage 16.

When the flexible printed board 20T is connected to the terminal portion10 a, the ground terminal FbG1, the ground terminal FbG2, the groundterminal FbG3, the ground terminal FbG4, and the ground terminal FbG5 ofthe flexible printed board 20T are respectively connected to the groundterminal TaG1, the ground terminal TaG2, the ground terminal TaG3, theground terminal TaG4, and the ground terminal TaG5 of the terminalportion 10 a. In addition, the differential signal terminal FbS1, thedifferential signal terminal FbS2, the differential signal terminalFbS3, and the differential signal terminal FbS4 of the flexible printedboard 20T are also connected to the differential signal terminal TaS1,the differential signal terminal TaS2, the differential signal terminalTaS3, and the differential signal terminal TaS4, respectively. Theconnection between the ground terminals and the connection between thesignal terminals are performed by soldering, for example.

FIG. 7 is a perspective view of the substrate 16 a of the optical module10T according to an embodiment of the present disclosure. In FIG. 7 ,the insulating layer of the substrate 16 a is transparent and the wiringlayer and the via holes are drawn with solid lines. FIG. 8 is anenlarged top view of region A of FIG. 6 for the terminal portion 10 a ofthe optical module 10T according to an embodiment of the presentdisclosure. Specifically, FIG. 8 is a top view when the terminal portionof the optical module 10T is viewed from the positive region of theY-axis toward the negative region of the Y-axis. In FIG. 8 , theinsulating layer of the substrate 16 a is transparent, the wiring layeris drawn with a solid line, and the via holes are drawn with dashedlines.

FIG. 9 is a cross-sectional view taken along the IX-IX line of FIG. 8 .Specifically, the terminal portion 10 a of the optical module 10T is cutalong the IX-IX line of FIG. 8 and is viewed from the positive region ofthe X-axis toward the negative region of the X-axis. In FIG. 9 , theinsulating layer of the substrate 16 a is transparent, and the wiringlayer and the via holes are indicated by solid lines.

The substrate 16 a includes a first wiring layer L1 provided on theupper surface 16 a 5 of the substrate 16 a, a second wiring layer L2formed under the first wiring layer L1, a first insulating layer Li1sandwiched between the first wiring layer L1 and the second wiring layerL2, a third wiring layer L3 formed under the second wiring layer L2, anda second insulating layer Li2 sandwiched between the second wiring layerL2 and the third wiring layer L3. That is, the third wiring layer L3,the second insulating layer Li2, the second wiring layer L2, the firstinsulating layer Li1, and the first wiring layer L1 are stacked in thisorder along the Y-axis direction from the negative region of the Y-axisto the positive region of the Y-axis. The first wiring layer L1, thesecond wiring layer L2, and the third wiring layer L3 are formedparallel to the XZ plane. The first wiring layer L1, the second wiringlayer L2, and the third wiring layer L3 are formed of a conductive metal(for example, tungsten). The first wiring layer L1 is the uppermostlayer of the substrate 16 a having a stacked structure, the secondwiring layer L2 is formed below the first wiring layer L1, and the thirdwiring layer L3 is formed below the second wiring layer L2.

(First Wiring Layer L1)

The substrate 16 a has the ground terminal TaG1, the ground terminalTaG2, the ground terminal TaG3, the ground terminal TaG4, and the groundterminal TaG5 as ground terminals in the first wiring layer L1. Inaddition, the substrate 16 a has the signal terminal TaS1 p, the signalterminal TaS1 n, the signal terminal TaS2 p, the signal terminal TaS2 n,the signal terminal TaS3 p, the signal terminal TaS3 n, the signalterminal TaS4 p, and the signal terminal TaS4 n as signal terminals inthe first wiring layer L1. The signal terminal TaS1 p and the signalterminal TaS1 n are used to transmit one differential signal. FIG. 6shows an exemplary embodiment in which four differential signals S1, S2,S3, and S4 are transmitted in parallel with one another. Onedifferential signal or one differential signal wiring is also referredto as a channel. That is, FIG. 6 illustrates an example embodiment inwhich four channels are handled. When only one channel is handled, thefirst wiring layer L1 may include at least the ground terminal TaG1, thesignal terminal TaS1 p, the signal terminal TaS1 n, and the groundterminal TaG2. In this case, the ground terminal TaG1, the signalterminal TaS1 p, the signal terminal TaS1 n, and the ground terminalTaG2 may be configured to have the above-described GSSG configuration.When two channels are handled, the first wiring layer L1 may include atleast the ground terminal TaG1, the signal terminal TaS1 p, the signalterminal TaS1 n, the ground terminal TaG2, the signal terminal TaS2 p,the signal terminal TaS2 n, and the ground terminal TaG3. In this case,the ground terminal TaG1, the signal terminal TaS1 p, the signalterminal TaS1 n, the ground terminal TaG2, the signal terminal TaS2 p,the signal terminal TaS2 n, and the ground terminal TaG3 may beconfigured to have the above-described GSSGSSG configuration.

Each of the ground terminals and the signal terminals is formed of aconductive member. Each of the ground terminals and the signal terminalsextends along the Z-axis direction. The ground terminal has a lateralwidth W1 (first width, see FIG. 10 ) in the X-axis direction. A lateralwidth of the signal terminal, the distance between the pair of signalterminals, and the distance between the signal terminal and the groundterminal are determined so that the characteristic impedance of thedifferential transmission line becomes a predetermined value accordingto a thickness of the insulating layer in the Y-axis direction, arelative dielectric constant of the insulating layer, and the like. Onthe other hand, for size reduction or high-density mounting of theoptical module 10T, the length of the terminal portion 10 a in theX-axis direction (for example, equal to a lateral width W of the opticalmodule 10T) is preferably a small value (see FIG. 6 ). Therefore, it ispreferable that the lateral width W1 of the ground terminal, the lateralwidth of the signal terminal, the distance between the pair of signalterminals, and the distance between the signal terminal and the groundterminal have small values.

(First Insulating Layer Li1)

The substrate 16 a includes the first insulating layer Li1 between thefirst wiring layer L1 and the second wiring layer L2. The firstinsulating layer Li1 is, for example, shaped as a green sheet includinga ceramic material, and is formed by sintering. The first insulatinglayer Li1 is formed of an insulating material and electrically insulatesthe first wiring layer L1 and the second wiring layer L2 from eachother. A thickness t1 of the first insulating layer Li1 is for example100 to 300 micrometers. The substrate 16 a is provided with via holespenetrating the first insulating layer Li1 in order to electricallyconnect a wiring or a terminal provided in the first wiring layer L1 toa wiring provided in the second wiring layer L2. The via holes will bedescribed later.

(Second Wiring Layer L2)

The substrate 16 a has a ground pattern TbG1 in the second wiring layerL2 under the ground terminal TaG1 of the first wiring layer L1.Similarly, the substrate 16 a has a ground pattern TbG2 under the groundterminal TaG2, a ground pattern TbG3 under the ground terminal TaG3, aground pattern TbG4 under the ground terminal TaG4, and a ground patternTbG5 under the ground terminal TaG5. The ground pattern has a lateralwidth W2 (second width, see FIG. 10 ) in the X-axis direction. Thelateral width W1 of the ground terminal is smaller than the lateralwidth W2 of the ground pattern. For example, as shown in FIG. 10 , theground terminal TaG3 is included in the ground pattern TbG3 in the planview from the Y-axis direction.

In the second wiring layer L2, a conductive member such as an electrodeis not provided below each of the signal terminal TaS1 p, the signalterminal TaS1 n, the signal terminal TaS2 p, the signal terminal TaS2 n,the signal terminal TaS3 p, the signal terminal TaS3 n, the signalterminal TaS4 p, and the signal terminal TaS4 n. In the region wherethere is no conductive member such as an electrode, the first insulatinglayer Li1 and the second insulating layer Li2 are formed in contact witheach other. In the region where there is no conductive member such as anelectrode, the first insulating layer Li1 and the second insulatinglayer Li2 may be integrated with each other.

(Second Insulating Layer Li2)

The substrate 16 a has the second insulating layer Li2 between thesecond wiring layer L2 and the third wiring layer L3. The secondinsulating layer Li2 is formed, for example, by shaping and sintering asa green sheet containing a ceramic material. The second insulating layerLi2 is formed of a material having insulating properties andelectrically insulates the second wiring layer L2 and the third wiringlayer L3 from each other. A thickness t2 of the second insulating layerLi2 is for example from 100 to 300 micrometers. The second insulatinglayer Li2 is provided with via holes for electrically connecting thewiring provided in the second wiring layer L2 and the wiring provided inthe third wiring layer L3. The via holes will be described later.

(Third Wiring Layer L3)

The substrate 16 a has a ground pattern TcG provided in the third wiringlayer L3 over the X-axis direction from below the ground terminal TaG1to below the ground terminal TaG5. Thus, the ground pattern TcG extendsalong the Y-axis Z-axis direction and the X-axis direction, or extendsin the XZ plane parallel to both the X direction and the Z direction.The ground pattern TcG may be a so-called solid pattern (solid ground).In the plan view from the Y-axis direction, the ground terminals TaG1,TaG2, TaG3, TaG4, and TaG5 are included in the ground pattern TcG. Thesubstrate 16 a may further include an insulating layer between the thirdwiring layer L3 and the lower surface of the substrate 16 a. In thiscase, the third wiring layer L3 is interposed between insulating layersin the Y-axis direction.

(Connection Structure Between First Wiring Layer L1 and Second WiringLayer L2)

Since the ground terminal TaG1, the ground terminal TaG2, the groundterminal TaG3, the ground terminal TaG4, and the ground terminal TaG5have the similar configuration, here, the ground terminal TaG3 will bedescribed as an example. The same applies hereinafter. The groundterminal TaG3 is provided between the differential signal terminal TaS2and the differential signal terminal TaS3 (see FIG. 6 ).

FIG. 10 is a top view of FIG. 8 with additional centerlines anddimensions for illustrative purposes.

The substrate 16 a includes via holes Va1, Va2, Va3, and Va4 (aplurality of first via holes) that electrically connect the groundterminal TaG3 provided in the first wiring layer L1 and the groundpattern TbG3 provided in the second wiring layer L2. Each of the viaholes Va1, Va2, Va3 and Va4 is formed to penetrate the first insulatinglayer Li1 along the Y-axis direction. The via hole Va1, the via holeVa2, the via hole Va3, and the via hole Va4 are holes provided from thefirst wiring layer L1 to the second wiring layer L2, and areelectrically connected to the first wiring layer L1 and the secondwiring layer L2 by plating the inside thereof. Note that each via holemay be filled with a metal. The via hole Va1, the via hole Va2, the viahole Va3, and the via hole Va4 are, for example, blind via holes. Oneend of the blind via hole in the Y-axis direction is exposed on thesurface 16 aS of the substrate 16 a. Another end of the blind via holeis inside the substrate 16 a and invisible from the outside. The insideof the via hole may be filled with a conductive metal. In a plan viewfrom the Y-axis direction, respective centers of the via hole Va1, thevia hole Va2, the via hole Va3, and the via hole Va4 are arranged sideby side on a centerline CL1 extending in the Z-axis direction of each ofthe ground terminal TaG3 and the ground pattern TbG3.

The via hole Va1, the via hole Va2, the via hole Va3, and the via holeVa4 are arranged such that their respective centers are along thecenterline CL1 at equal intervals each of which is an interval pa (firstinterval). The centerline CL1 is a virtual straight line that extendsalong the Z-axis direction in a plan view viewed from the Y-axisdirection. The interval pa is, for example, a distance between thecenter of the circle of the via hole Va1 and the center of the circle ofthe via hole Va2.

(Connection Structure Between Second Wiring Layer L2 and Third WiringLayer L3)

The substrate 16 a includes via holes Vb11, Vb12, Vb13, and Vb14 (aplurality of second via holes) that connect the ground pattern TbG3provided in the second wiring layer L2 and the ground pattern TcGprovided in the third wiring layer L3. Each of the via holes Vb11, Vb12,Vb13, and Vb14 is formed to penetrate the second insulating layer Li2along the Y-axis direction. The via hole Vb11, the via hole Vb12, thevia hole Vb13, and the via hole Vb14 are holes provided from the secondwiring layer L2 to the third wiring layer L3, and inner sides thereofare plated to be electrically connected to the second wiring layer L2and the third wiring layer L3. Note that each via hole may be filledwith a metal. The via hole Vb11, the via hole Vb12, the via hole Vb13,and the via hole Vb14 are, for example, buried via holes. Both ends ofthe buried via hole in the Y-axis direction are buried in the substrate16 a and invisible from the outside. In addition, the substrate 16 aincludes via holes Vb21, Vb22, Vb23, and Vb24 (a plurality of third viaholes) that connect the ground pattern TbG3 provided in the secondwiring layer L2 and the ground pattern TcG provided in the third wiringlayer L3. The via holes Vb21, Vb22, Vb23, and Vb24 are formed topenetrate the second insulating layer Li2 along the Y-axis direction.The via hole Vb21, the via hole Vb22, the via hole Vb23, and the viahole Vb24 are holes provided from the second wiring layer L2 to thethird wiring layer L3, and inner sides thereof are plated to beelectrically connected to the second wiring layer L2 and the thirdwiring layer L3. Note that each via hole may be filled with a metal. Thevia hole Vb21, the via hole Vb22, the via hole Vb23, and the via holeVb24 are, for example, buried via holes.

The via hole Vb11, the via hole Vb12, the via hole Vb13, and the viahole Vb14 are arranged at equal intervals each of which is an intervalpb1 (second interval), when viewed along a centerline CL21. Thecenterline CL21 is a virtual straight line that extends along the Z-axisdirection in the plan view viewed from the Y-axis direction. Theinterval pb1 is, for example, the distance between the center of thecircle of the via hole Vb12 and the center of the circle of the via holeVb13. The centerline CL21 is provided away from the centerline CL1 by aninterval d1 (first distance) to be on the negative side of the X-axisdirection in a plan view viewed from the Y-axis direction (when viewedfrom the positive side to the negative side, in the Y-axis direction).Therefore, the centerline CL21 is provided between the centerline CL1and the differential signal terminal TaS2.

The via hole Vb21, the via hole Vb22, the via hole Vb23, and the viahole Vb24 are arranged at equal intervals each of which is an intervalpb2, when viewed along a centerline CL22. The centerline CL22 is avirtual straight line that extends along the Z-axis direction, in theplan view viewed from the Y-axis direction. The interval pb2 is, forexample, the distance between the center of the circle of the via holeVb22 and the center of the circle of the via hole Vb23. The centerlineCL22 is provided away from the centerline CL1 by an interval d2 (seconddistance) to be on the positive side of the X-axis direction in a planview viewed from the Y-axis direction (when viewed from the positiveside to the negative side, in the Y-axis direction). Therefore, thecenterline CL22 is provided between the centerline CL1 and thedifferential signal terminal TaS3. Note that the interval d2 may be setto the same distance as the interval d1. The via hole Vb21, the via holeVb22, the via hole Vb23, and the via hole Vb24 (the plurality of thirdvia holes) and the via hole Vb11, the via hole Vb12, the via hole Vb13,and the via hole Vb14 (the plurality of second via holes) are spacedapart from one another to be at an offset by a distance that is half theinterval pb1 or half the interval pb2 when viewed along the Z-axisdirection. Note that the interval pb2 may be set to be the same distanceas the interval pb1.

In the optical module 10T according to the embodiment of the presentdisclosure, the interval pb1 and the interval pb2 may be set to the samedistance as the interval pa. It is preferable that the interval pa, theinterval pb1, and the interval pb2 be made as small as possible inaccordance with, for example, the accuracy of drilling of the via hole,the thickness and strength of the insulating layer, and the like. Forexample, the interval pa, the interval pb1, and the interval pb2 are setto 200 to 350 micrometers. In the optical module 10T according to theembodiment of the present disclosure, by arranging the via holes Vb21,Vb22, Vb23, and Vb24 (the plurality of third via holes) and the viaholes Vb11, Vb12, Vb13, and Vb14 (the plurality of second via holes) soas to be mutually displaced, the apparent effective interval of the viaholes provided between the differential signal terminal TaS2 and thedifferential signal terminal TaS3 can be narrowed. The effectiveinterval is an apparent interval of the via holes as viewed along the Xdirection.

For example, when the interval pa, the interval pb1 and the interval pb2are 300 micrometers and the diameter of the via hole is 75 micrometers,the interval between the via hole Vb11 and the via hole Vb21 in FIG. 9is 75 micrometers. For example, the wavelength of 80 gigahertzelectromagnetic wave propagating through the insulating layer is about 1mm to 2 mm. Therefore, since the apparent interval of the via holesbecomes sufficiently smaller than the quarter wavelength of theelectromagnetic wave propagating through the insulating layer, itbecomes difficult for the electric field generated by the signaltransmission to pass between the via holes, and the via holes Vb21,Vb22, Vb23 and Vb24 and the via holes Vb11, Vb12, Vb13 and Vb14 act as ashield between the differential signal terminal TaS2 and thedifferential signal terminal TaS3.

On the other hand, when the via holes are arranged without being shiftedfrom each other, the interval between the via holes becomes 225micrometers, and there is a possibility that the via holes do not act asa shield particularly at a high frequency. For example, when thedifferential signal includes frequency components equal to or higherthan the 80 GHz, an electric field generated by signal transmission maypass between the via holes to cause crosstalk between the differentialsignal terminal TaS2 and the differential signal terminal TaS3.

FIG. 11 shows a result of an electromagnetic field analysis calculatedfor crosstalk between differential signals when the optical module 10Taccording to the embodiment is used. FIG. 11 shows an electric fielddistribution when a differential signal is input to the signal terminalprovided in a region Rtx. A region Rgnd indicates a region in which theground terminal is provided. A region Rct indicates a region in whichthe signal terminal is provided. A differential signal is not input tothe signal terminal of the region Rct. Although the differential signalinput to the signal terminal provided in the region Rtx is actuallytransmitted toward the right side along the Z-axis direction of FIG. 11, FIG. 11 shows only the result of the terminal portion 10 a. Morespecifically, only the results related to the via hole Vb12, the viahole Vb13, the via hole Vb21, the via hole Vb22, and the via hole Vb23are shown.

As shown in FIG. 11 , it is understood that the electric field generatedin the region Rtx is shielded in the region Rgnd. Therefore, althoughthe shade of color shading indicates the intensity of the electric fieldof the differential signal, the spread of the electric field in theX-axis direction is suppressed by the ground terminal in the regionRgnd, and the generation of the electric field in the region Rct is notseen.

Here, as the optical module according to the comparative example, FIG.12 shows a result of electromagnetic field analysis performed on a casewhere via holes connecting the ground pattern of the second wiring layerL2 and the ground pattern of the third wiring layer L3 are arranged in aline in the Z-axis direction in the terminal portion 10 a. Morespecifically, two white circles in FIG. 12 indicate a via holeconnecting the second wiring layer L2 and the third wiring layer L3provided immediately below the via hole Va2 and a via hole connectingthe second wiring layer L2 and the third wiring layer L3 providedimmediately below the via hole Va3. The via holes Va2 and Va3 connectingthe first wiring layer L1 and the second wiring layer L2 are omitted inFIG. 12 because they are the same as the two black circles in FIG. 11 .

As shown in FIG. 12 , it can be seen that the electric field generatedin the region Rtx spreads in the X-axis direction, passes through theregion Rgnd, and leaks out to the region Rct. That is, the result ofFIG. 12 indicates that in the optical module according to thecomparative example, crosstalk may occur between the differentialsignals transmitting in the signal terminals adjacent to each other.

By shifting the via holes Vb21, Vb22, Vb23, and Vb24 arranged along theZ-axis direction from the via holes Vb11, Vb12, Vb13, and Vb14 arrangedalong the Z-axis direction by the half distance of the interval pb1along the Z-axis, so that the centers of the via holes Vb21, Vb22, andVb12 form an equilateral triangle in a plan view from the Y-axisdirection (see FIG. 13A). At this time, by arranging the two rows of viaholes so as to be offset from each other along the Z-axis direction, aninterval pc of the two rows of via holes can be made smaller than thevalue of the interval pa (=interval pb1=interval pb2) described above.For example, if two rows of via holes are arranged at the same positionin the Z-axis direction without being offset by half the interval pa inthe Z-axis direction, the interval pc of the two rows of via holes canbe equal to the interval pa but cannot be smaller than the interval pa(see FIG. 13B). By making the interval pc smaller than the interval pa,the lateral width W2 of the ground pattern TbG3 of the second wiringlayer L2 can be reduced.

According to the package 16 of the optical module 10T according to theembodiment of the present disclosure, it is possible to prevent anelectric field generated in one signal terminal by a differential signalfrom leaking from a via hole of a ground terminal adjacent to the signalterminal and being transmitted to a differential signal transmittedthrough another signal terminal adjacent to the ground terminal. Thatis, the optical module according to the embodiment of the presentdisclosure can suppress crosstalk between signals transmitted throughsignal terminals adjacent to each other with a ground terminalinterposed therebetween.

Crosstalk between signals is suppressed by via holes in ground terminalsbetween adjacent signal terminals. For example, by providing via holesat a narrow pitch (interval), crosstalk can be suppressed up to afrequency component of a higher frequency included in a signal(shielding effect due to a ground terminal). However, there is alimitation on the pitch of via holes in terms of package manufacturing.In mass production, if the pitch of the via holes cannot be madenarrower than the limitation on package manufacturing, the shieldingeffect becomes insufficient for a signal having a very high signalspeed, and crosstalk may increase.

In the package 16 of the optical module 10T according to the embodimentof the present disclosure, the via hole Vb21, the via hole Vb22, the viahole Vb23, and the via hole Vb24 arranged along the Z-axis direction andthe via hole Vb11, the via hole Vb12, the via hole Vb13, and the viahole Vb14 arranged along the Z-axis direction are arranged to be shiftedfrom each other by half the distance of the interval pb1 in the Z-axisdirection. By arranging the via holes so as to be shifted from eachother by a half distance, the apparent pitch of the via holes whenviewed from the X-axis direction can be made smaller than the limitationon package manufacturing. Therefore, with the package 16 of the opticalmodule 10T according to the embodiment of the present disclosure, it ispossible to suppress crosstalk between signals transmitted adjacent toeach other up to frequency components of higher frequencies included inthe signals. As a result, a higher-speed signal can be transmitted tothe optical module 10T.

Further, in the package 16 of the optical module 10T according to theembodiment of the present disclosure, the via hole Va1, the via holeVa2, the via hole Va3, and the via hole Va4 are arranged at equalintervals in a row along the centerline CL1 in which the centers of thevia holes Va1, Va2, Va3, and Va4 extend in the z-axis direction.Therefore, in the package 16 of the optical module 10T according to theembodiment of the present disclosure, the width of the ground terminalof the first wiring layer L1 can be set to a minimum value due tomanufacturing constraints. Thereby, for example, with respect to theGSSG configuration of the terminals, it is possible to set thecharacteristic impedance of the differential signal SS to a desiredvalue and to set the distance (interval) between GG of both ends to theminimum value of the restriction in manufacturing. This makes itpossible to minimize the length of the terminal portion 10 a in theX-axis direction (corresponding to the lateral width W in FIG. 3 ),which is suitable for size reduction of the optical module 10T.

For example, by increasing the width of the ground terminal, the firstwiring layer L1 and the second wiring layer L2 may be connected by tworows of via holes arranged along the Z-axis direction, respectively.However, when two rows of via holes are arranged at the same positionalong the Z-axis direction, the width of the ground terminal becomeswider than a width of a ground terminal including only one row of viaholes by the interval of the via holes. It is difficult to increase thewidth of the ground terminal when the length of the package 16 in theX-axis direction (see the lateral width W in FIG. 6 ) is limited. Inaddition, although the shapes and intervals of the terminal portionssuch as the signal terminals and the ground terminals are designed inconsideration of the characteristic impedance of the transmission lineconfigured by the signal terminals and the ground terminals, there is apossibility that a desired characteristic impedance cannot be obtainedby increasing the width of the ground terminal.

According to the package 16 of the optical module 10T according to theembodiment of the present disclosure, by suppressing an increase in thelateral width W2 of the ground pattern of the second wiring layer, thevalue of the desired characteristic impedance can be maintained within arange in which the influence on the high-frequency characteristics issmall, and crosstalk between signals adjacent to each other can besuppressed. For example, it is assumed that a differentialcharacteristic impedance is Z0 when the via holes between the firstwiring layer L1 and the second wiring layer L2 and the via holes betweenthe second wiring layer L2 and the third wiring layer L3 are arranged inone row (comparative example 1). A differential characteristic impedanceZ1 of the package 16 of the optical module 10T according to theembodiment of the present disclosure having one row of via holes betweenthe first wiring layer L1 and the second wiring layer L2 and two rows ofvia holes between the second wiring layer L2 and the third wiring layerL3 is calculated to be 0.7% smaller than the differential characteristicimpedance Z0 (see FIG. 13A for the arrangement of the via holesconnecting the ground pattern of the second wiring layer L2 and theground pattern of the third wiring layer L3). On the other hand, whenthe via holes between the first wiring layer L1 and the second wiringlayer L2 and the via holes between the second wiring layer L2 and thethird wiring layer L3 are arranged in two rows (comparative example 2),a differential characteristic impedance Z2 is calculated to be smallerthan the differential characteristic impedance Z0 by 3.9% (see FIG. 13Bfor the arrangement of the via holes connecting the ground pattern ofthe second wiring layer L2 and the ground pattern of the third wiringlayer L3). As described above, the two rows of via holes connecting theground pattern of the second wiring layer L2 and the ground pattern ofthe third wiring layer L3 are arranged so as to be shifted from eachother by a distance of a half of the interval pa along the Z-axisdirection, whereby a change in the differential characteristic impedanceZ2 can be reduced.

The above-described analysis results are obtained because the width ofthe ground terminal of the first wiring layer L1 is the same as that ofthe comparative example 1 and the width of the ground pattern of thesecond wiring layer L2 is smaller than that of the comparative example2, so that an increase in the parasitic capacitance between the signalterminal and the ground is suppressed. Therefore, with the package 16 ofthe optical module 10T according to the embodiment of the presentdisclosure, it is possible to suppress a change in the value of thecharacteristic impedance and to suppress crosstalk between signalstransmitted adjacent to each other.

The Z-axis direction is an example of a first direction, and the X-axisdirection is an example of a second direction that intersects the firstdirection. For example, when the ground terminal TaG3 is exemplified asthe first ground terminal, the differential signal terminal TaS2 is anexample of the first signal terminal, and the differential signalterminal TaS3 is an example of the second signal terminal. Further, theground pattern TbG3 is an example of a first ground pattern, and theground pattern TcG is an example of a second ground pattern.

The via holes Va1, Va2, Va3, and Va4 are examples of first via holes,the via holes Vb11, Vb12, Vb13, and Vb14 are examples of second viaholes, and the via holes Vb21, Vb22, Vb23, and Vb24 are examples ofthird via holes. The interval pa is an example of first interval. Theinterval pb1, and the interval pb2 are examples of first secondinterval.

<Variation>

Although the optical module 10 according to the embodiment of thepresent disclosure has been described using the optical module 10T as anexample of the TOSA, the optical module 10 is not limited to the TOSA.For example, the optical module 10 may be the optical module 10R as anexample of a ROSA. Further, for example, the optical module 10 may be anoptical module in which a light source, an optical modulator, and anoptical receiver are accommodated in one package. That is, the package16 can be applied as various packages for optical modules that input andoutput a plurality of high-speed signals.

In the optical module 10 according to the embodiment of the presentdisclosure, although the signal transmitted by the signal terminal isdescribed as a differential signal, the signal transmitted to the signalterminal may be a single-ended signal.

What is claimed is:
 1. A package for an optical module comprising: aside wall; and a substrate provided through the side wall in a firstdirection, the substrate including a first wiring layer that isexternally exposed, the first wiring layer including a first signalterminal extending along the first direction, a second signal terminalextending along the first direction, and a first ground terminalextending along the first direction, the first ground terminal beingprovided between the first signal terminal and the second signalterminal, when viewed in a second direction perpendicular to the firstdirection, a second wiring layer disposed under the first wiring layer,the second wiring layer including a first ground pattern, the firstground pattern being electrically coupled to the first ground terminalvia a plurality of first vias, a third wiring layer disposed under thesecond wiring layer, the third wiring layer including a second groundlayer electrically coupled to the first ground pattern via a pluralityof second vias and a plurality of third vias; a first insulating layerdisposed between the first wiring layer and the second wiring layer, thefirst insulating layer including the plurality of first vias, theplurality of first vias being arranged at a first interval along thefirst direction, and a second insulating layer disposed between thesecond wiring layer and the third wiring layer, the second insulatinglayer including the plurality of second vias, the second vias beingarranged at a second interval along the first direction, and theplurality of third vias, the third vias being arranged at the secondinterval along the first direction, wherein when viewed in the seconddirection, each first via of the plurality of first vias is providedbetween a corresponding second via among the plurality of second viasand a corresponding third via among the plurality of third vias, andwherein when viewed in the first direction, each second via of theplurality of second vias is disposed at an offset by half of the secondinterval from the corresponding third via among the plurality of thirdvias.
 2. The package according to claim 1, wherein a first via of theplurality of first vias and the corresponding second via of theplurality of second vias of the substrate are separated by a firstdistance, wherein the corresponding second via of the plurality ofsecond vias and the corresponding third via of the plurality of thirdvias of the substrate are separated by a second distance, and whereinthe second distance is set to be equal to the first distance in thesecond direction.
 3. The package according to claim 1, wherein the firstground terminal has a first width in the second direction, and the firstground pattern has a second width in the second direction, and whereinthe first width is narrower than the second width.
 4. The packageaccording to claim 1, wherein a centerline of the first ground terminalextends along the first direction, and wherein the first vias arearranged so that centers of the first vias are on the centerline of thefirst ground terminal, when viewed in the first direction.
 5. Thepackage according to claim 4, wherein the centerline of the first groundterminal and the first signal terminal are separated by a first distancein the second direction, and wherein the centerline of the first groundterminal and the second signal terminal are separated by a seconddistance in the second direction, and wherein the second distance is setto be equal to the first distance.
 6. An optical module comprising: thepackage according to claim 1; and an optical semiconductor deviceincluded in the package.
 7. A flexible printed board configured to becoupled to the package of the optical module according to claim
 6. 8. Anoptical module assembly comprising: the optical module according toclaim 6; and a flexible printed board coupled to the optical module.